IR Spectroscopy: Identifying Double and Triple Bonds

What material are glowers in IR spectrometers typically made of?

Which type of monochromator has become obsolete in IR spectrometers?

What type of detector is used in an IR spectrometer?

Which of the following can be used as a sample in an IR spectrometer?

What is the typical concentration range for a solid sample in a KBr disk in an IR spectrometer?

Which absorption range is indicative of a C=C double bond?

What absorption region often implies the presence of an aromatic ring?

A sharp absorption near 2250 cm-1 is commonly associated with which functional group?

Which absorption region is characteristic of a nitro group?

If none of the characteristic absorptions for double, triple bonds or nitro groups are found, which absorptions would you expect to find in a hydrocarbon?

Functional Groups in IR Spectroscopy

Double bonds and/or aromatic rings

C=C bonds absorb weakly near 1650 cm-1
Medium-to-strong absorptions between 1600-1450 cm-1 often indicate aromatic rings
Aromatic and vinyl C-H absorbances occur left of 3000 cm-1, distinguishing them from aliphatic C-H

Triple bonds

C≡N bonds absorb medium, sharply near 2250 cm-1
C≡C bonds absorb weakly, sharply near 2150 cm-1
Acetylenic C-H absorbs near 3300 cm-1

Nitro groups

Two strong absorptions between 1600-1530 cm-1 and 1390-1300 cm-1

Hydrocarbons

No absorptions characteristic of other functional groups
Major absorptions in C-H region near 3000 cm-1
Simple spectrum with additional absorptions near 1460 and 1375 cm-1

IR Spectrometer Components

Light Source

Globar (electrically heated silicon carbide rod)
Nernst glower (small heated refractory oxide rod)

Monochromators

Prism (obsolete)
Grating (better resolution, especially at high frequency)

Detector

Thermocouple detector

IR Spectroscopy Process

Light passes through a sample, and frequencies matching vibrational frequencies are absorbed
Remaining light is transmitted to a detector

Sample Preparation

Pure liquid samples (e.g., acetone, chloroform, methanol)
Solid in liquid (dissolve solute in solvent)
Solid sample in KCl or KBr disk (transparent disk, 0.1-0.2% concentration, e.g., 2 mg sample in 200 mg KBr)

Learn to identify double bonds, aromatic rings, and triple bonds in infrared spectroscopy through characteristic absorption patterns. Understand how to confirm these bonds by analyzing the C-H region. A must-know for organic chemistry students!